An on-ramp to an expressway sometimes directly leads to an off-ramp, if a lane is not changed to a main lane just after passing through a converging point with the expressway. To prevent directly entering the off-ramp, a navigation system is proposed that conducts route guidance at a branching point that branches into both of a main lane of the expressway and an off-ramp (Patent Document 1).
Patent Document 1: JP-2000-266556 A (U.S. Pat. No. 6,466,867 B1)
However, in a case that a distance from the converging point to the branching point is short, the route guidance after passing through the converging point may give a user no sufficient time period to determine and operate a lane change. The similar problem may occur also in a guidance intersection. This guidance intersection is referred to as an intersection at which, to follow the route guidance, a subject vehicle needs to conduct turning right or left or the like instead of continuing traveling a road the vehicle is traveling.
For instance, suppose a case that a vehicle turns left by following the route guidance or a guidance route and, just after turning left, suggested that a lane be changed to the rightmost lane so as to follow the guidance route. In this case, the above problem of giving a user no sufficient time period to determine and operate a lane change may also occur.